The general idea of a combined vehicle starter-generator that can operate in both a starting or cranking mode and in a generating mode has been known for some time. Such a mechanism, which may be conveniently referred to as a "stargen", promises substantial weight and cost savings, since the separate starter motor could be eliminated. However, the stargen has not found wide application because of several practical problems presented by its actual use. One of these practical problems is in maintaining the proper tension in the belt. A belt tensioner should operate on the slack side of the belt, which is not a problem in a conventional installation, since the slack side of the belt is invariant. With a stargen, however, since the belt would be driven in the starting mode, but would be the driver in the generating mode, the slack and tight sides of the belt would consequently shift with the change in mode.